Production of sodium silicofluoride from wet process phosphoric acid



Sept. 15, 1970 w. A. SATTERWHITE EI'AL' 3,528,786 PRODUCTION OF SODIUM SILICOFLUORIDE FROM WET PROCESS PHOSPHORIC ACID Filed Sept. 5, 1969 I0 Nh SiF DISSOLUTION max m s"; mo HOT WATER I ovERFww I msuu'rso TRAP ron UNDISSOLVED No SiF ovsnnow I I3 l BAFFLED CRYSTALLIZING rnouen :0

WATER COOLING JACKET :6 nscvcu: vmsn INVENTORS:

WILLIAM A. SATTERWHITE JACK M. CARTER a. a rr'v United States Patent Office 3,528,786 PRODUCTION OF SODIUM SILICOFLUORIDE FROM WET PROCESS PHOSPHORIC ACID William A. Satterwhite and Jack M. Carter, Lakeland, Fla., assignors to United States Steel Corporation, a corporation of Delaware Continuation-impart of application Ser. No. 546,877, May 2, 1966. This application Sept. 5, 1969, Ser. No. 855,730

Int. Cl. B0111 9/00; G01b 33/10 U.S. Cl. 23-302 9 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of application Ser. No. 546,877, filed May 2, 1966, now abandoned.

This invention relates to the production of sodium silicofiuoride from wet process pohsphoric acid, and more particularly to the preparation of the product in crystals of large size.

Wet process phosphoric acid contains as impurities fluorine, fluosilicates, aluminum and iron salts, and other impurities. Such acid, as, for example, 30 weight percent P O orthophosphoric acid or similar feed acid, may be treated to precipitate sodium silicofiuoride and the precipitate may be separated by centrifuge or filtration to provide a crude sodium silicofiuoride product. For example, a sodium ion, such as is provided by sodium sulfate, sodium carbonate, etc., may be introduced into the wet process feed acid to ring about a precipitation of sodium silicofiuoride and calcium sulfate. The recovered sodium silicofiuoride product, however, presents a problem because of its high P content, which exceeds the specifications required by purchasers and because treatment of the crude product results in a fine powdery purities.

We have discovered that by controlled treatment of the precipitated sodium silicofiuoride product under high temperatures and slow cooling that a large crystal product can be obtained in quantity and in few steps and with the P 0 content below the acceptable level of 0.06 percent by weight. By utilizing pressure and high temperatures in the dissolving of the sodium silicofiuoride starting material in water containing less than 1 percent by weight of impurities and thereafter cooling slowly, large crystals can be obtained which are readily recovered as a high purity product. The water employed must contain less than 1 percent by weight of impurities in order to avoid excessive common ions and other factors that inter-,

fere with dissolution of the sodium silicofiuoride.

A primary object, therefore, of the invention is to provide a process for the recovery of crystalline silicofiuoride substantially fre of P 0 content and of other impurities. A further object is to provide a process by which precipitated sodium silicofiuoride is dissolved in water at high temperatures and then recrystallized to obtain a large granular crystal of chemically pure sodium silicofiuoride. A still further object is to provide a process in which high temepartures and pressures are employed in 3,528,786 Patented Sept. 15 1970 the treatment of precipitated sodium silicofiuoride followed by slow cooling to bring about a recovery of crystals in large size. Other specific objects and advantages will appear as the specification proceeds.

In one embodiment of our invention, sodium silicofluoride is recovered as a precipitate from wet process feed orthophosphoric acid, such as, for example, 30 Weight percent P 0 acid by adding sodium ions such as is provided by sodium sulfate, sodium carbonate, etc., and the precipitated sodium silicofluride is passed through a centrifuge, as, for example, a bowl centrifuge for the separation of the sodium silicofiuoride. The sodium silicofiuoride is then pulped with water and heated in a vessel at a temperature of at least 98 C. and preferably with gentle agitation. After the leaching of the mother liquor with the hot water, the material is cooled slowly to bring about the formation of large crystals.

The process is illustrated in one embodiment by the accompanying drawing in which apparatus useful in the carrying out of the process is shown diagrammatically. 10 indicates a sodium silicofiuoride dissolution tank which is preferably closed to permit the building up of pressure therein. A portion of the tank is cut away to show an agitator 11 within the tank, the agitator being driven by motor 12. Sodium silicofiuoride and hot water are introduced into the tank and gently agitated therein to bring about dissolving of the sodium silicofiuoride. Tank 10 may, if desired, be heated. We desire to maintain a temperature of at least 98 C. and preferably higher. By utilizing superatmospheric pressure, higher temperatures which are found to be most effective in bringing about increased solubility of the sodium silicofiuoride and thereby increasing the production rate of the product can be employed.

through line 18 to the tank 10.

To promote crystallization within the trough 15, We provide metal 'baflles 19 on which the sodium silicofiuoride forms crystals. For example, metal surfaces are desired for crystallization of the large particle size sodium silicofiuoride. The metal bafiles provide surfaces on v stals clin in order to nucleate and grow. material wh1ch cannot be effectively separated from imthe cry Slow cooling of the saturated water solution of sodium silicofiuoride is desired for large crystal growth. For example, when the sodium silicofiuoride is dissolved in tank 10'at a 'temperature'of about 98 C. and discharged from the trap into the crystallizing trough at a temperature of around 80 C., we prefer a retention time inthe trough of 10 minutes and more with a discharge end temperature of around C. In other words, by employing a 20-30 C. temperature drop through a period of 10 minutes to about 2 hours, we find that the crystals are formed in substantial quantity and in large size.

By employing pressures in tank 10 of about 10 to 100 pounds per'square inch,'and preferably between 20 and 60 pounds per. square inch, we find that temperatures can i be employed in the range of 100-250 C., and that as a While slow cooling is necessary, the extent of the retening to an undissolved sodium silicofluoride trap also of tion time during cooling for crystallization may vary substainless steel construction and having a 6 gallon capacity stantially. Flash cooling by the use of refrigeration was and closed as a pressure vessel for pressures up to 60 found not to produce large crystals. However, a substanpounds per square inch. The vessel is insulated. tial amount of large crystals can be produced even when 5 The overflow from the trap vessel is to a stainless steel the retention time is relatively short, say, about mintrough 24 feet long, 6 inches wide and 6 inches high, utes. We prefer, however, a retention time of around 20 fitted with a mild steel water cooling jacket, with 2 inches minutes, and this period may extend up to 2 hours with of water bottom and sides of the crystallizer trough, The beneficial results. During the cooling period, we prefer trough is mounted in inclined position, with the end to keep the exit temperature in the crystallization zone at 10 nearest the dissolution tank 6 inches higher than the water at least 50 C. or above, and preferably above 60 C. It discharge end. The stainless steel trough has metal bafiles will be understood that the temperatures during cooling every 6 inches along its length and is equipped with an will vary substantially depending upon the temperatures automatic means for removing the baffles every 30 minemployed in the dissolution tank, the impurities present, Utes and for p ing them With a Clean Set Of bafiles,

t means being also provided for stripping the product from High temperatures play a significant part in the produce removed baflles.

tion of the large crystals. The hot water leaching at 98 C. Y employing the foregoing apparatus and maintainand above followed by cooling results in the formation g t high temperature and pressure Conditions, the

of very large crystals 30 h) around h id f production rate of product is increased by at least fourthe reaction vessel just above the liquid level. These crys- 20 fOldtals amount to about 8 percent by weight of the total EXAMPLE IV silicofluoride processed and are of extremely high purity. The entire process results in the production of +65 mesh crystals in quantity.

Why the high temperatures are found so significant in the production of the large crystal product, We cannot explain with certainty. Over and above producing an increase in solubility, the high temperatures apparently have some effect upon the impurities or other factors which cause the formation of large crystals during the cooling period.

Specific examples illustrative of the process may be set out as follows:

The process was carried out as described in Example 1 except that instead of using an open tank 10, we employed a pressure cooked employing 15 pounds pressure. The sodium silicofluoride was reacted with water in a household pressure cooker for 30 minutes. Pressure was released quickly, the vessel opened, and a sample of the liquid taken as quickly as possible. The liquid sample was cololled to room temperature and analyzed for percent S01 s.

Solubility tests employing the pressure cooker were compared with solubility tests made with the open vessel tank employed in Example I. The folloWin Table III is EXAMPLE I a summary of the solubility tests and showi that a pres- A small crystallize was built as shown in the drawing, sure cooker can be expected to diSSOlVC almost twice as the vessel 10 having a 10 gallon capacity and the inmuch Sodium silicofluoride as an p Vessel. sulated trap for undissolved sodium silicofluoride having TABLE 111 a 2 gallon capacity. The vessel 10 was an open stainless steel tank, and the temperatures employed were 98-100" w C. The sodium silicofluoride at a temperature of 98-99" Test Number g s p e t te open C. was kept in gentle agitation with an overflow of silicos'plessme tank at 210 fluoride to the trap 13. The baflles were located every 12 g .4 inches. Feed end temperature was 82 C., and the dis- I:IIIIIIIIIIIIIIIIIIIIII:IIIIIII 2:14 138 charge end temperature was 60 C., the retention time in l; the crystallizer trough being 20 minutes. The water re- .I 2102 1Z0 cycle rate was 100 cc. per minute. The product rate was 0 /1 pound product per hour, the product being removed by While in the foregoing description we have described hand every 2 or 3 hours. The chemical analysis of the the precipitated sodium silicofluoride as being dissolved product is set out in the following Table I: directly in hot water, it may be desirable in some cases TABLE I Percent Material +05 Mesh P205 CaO so, F0203 K90 NflzsiFl N0./Ft;.

Feed 1.0 0.11 0. 20 0.59 0.06 0. 04 09.2 77.5 Product 76.9 0. 003 0.23 0.58 0. 03 0. 03 99.9 79.5

EXAMPLE II to first repulp the precipitated sodium silicofluoride in cold water for the purpose of dissolving some impurities The process was carried out as described 1n Example I and then filtering the material, the recovered silicofluoride with the product having the chemical analysis set out in solids being then passed to the hot water tank 10. Our Table II: preferred operation, however, is to pass the precipitated TABLE II Percent Material Mesh P205 08.0 S04 F8203 K20 NazSlFo No./Ft.

Feed 3.0 0. 0. 26 0. 50 0.07 0.07 98.9 78.6 Product 77.0 0. 03 0.21 0. 43 0.04 0. 04 99.5 78.3

EXAMPLE III sodium silicofluoride directly to the hot Water tank be- A stainless steel tank corresponding to tank 10 of the 70 at the Very high temPeratPres employed, the crystal drawing is provided in a 30 gallon capacity as a pressure 11116 producf can 'Obtamed subs'tantlauy P form vessel and is operated in 60 pounds per square inch emand flflls 'wlth mlnlmjflm 0f B g S p ploying temperatures ranging from 105250 C. The tank Wh1le in the foregoing specification we have set out is provided with a stainless steel agitator and is heated specific procedures in considerable detail for the purpose with steam coils. The overflow is as shown in the drawof illustrating embodiments of the invention, it will be understood that such details may be varied widely by those skilled in the art without departing from the spirit of our invention.

We claim:

1. In a process for the preparation of crystalline sodium silicofluoride from wet process orthophosphoric acid containing impurities including fiuosilicates and in which sodium ions are introduced into the wet process orthophosphoric acid to precipitate sodium silicofluoride and the precipitate recovered, the steps of dissolving the precipitate in hot water containing less than 1% by weight of impurities at a temperature of at least 98 C., and then slowly cooling the same to crystallize the sodium silicofluoride, said sodium silicofluoride product containing less than 0.05% by weight P 2. The process of claim 1 in which the precipitate is dissolved in water under superatmospheric pressure at a temperature in excess of 100 C.

3. The process of claim 1 in which the sodium silicofluoride is cooled slowly from a temperature of about 80 to about 60 C.

4. The process of claim 1 in which the precipitated sodium silicofluoride is first repulped in cold water and filtered before heating the silicofluoride.

5. In a process for the preparation of crystalline sodium silicofluoride of +65 mesh recovered as a precipitate, the steps of rissolving the precipitate in hot water containing less than 1% by weight of impurities at a temperature of at least 98 C. and under superatmospheric pressure, and slowly cooling the same during a period of at least minutes to a temperature not substantially below 50 C.

6. A process for the preparation of substantially pure sodium silicofluoride crystals of +65 mesh and containing less than about 0.05% by weight P 0 which comprises dissolving precipitated sodium silicofluoride containing impurities resulting from the introduction of sodium ions into wet process orthophosphoric acid in hot water containing less than 1% by weight of impurities at a temperature of at least 98 C. and forming said sodium silicofluoride crystals of +65 mesh by cooling the resulting solution to cause a to C. temperature drop over a period of about 10 minutes to about 2 hours.

7. The process of claim 6 wherein said cooling period is about 10 minutes to about 2 hours.

8. The process of claim 5 wherein said cooling period is about 10 minutes to about 2 hours.

9. The process of claim 1 wherein there is approximately a 20 to 30 C. temperature drop over a period of about 10 minutes to about 2 hours during said slow cooling to crystallize the sodium silicofluoride.

References Cited UNITED STATES PATENTS 2,447,359 8/1948 Oakley 23-88 2,790,705 4/1957 Kean et all. 2388 2,865,709 12/1958 Horn et al. 23--88 3,055,733 9/1962 Lang et a1. 2388 3,462,242 8/1969 Barker et all 23--88 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner US. Cl. X.R. 23S8 

